Repulpable silicone release liner paper with dissolvable layer

ABSTRACT

A method of producing and repulping paper having a silicone release liner includes producing paper having a silicone release liner that is initially secured to the paper by a dissolvable material. During production of the paper, a water-soluble coating may be applied to a paper surface, and a silicone release coating is then applied to the water-soluble coating. Upon rewetting of the paper during repulping, the water-soluble coating dissolves, thereby enabling separation of the silicone release coating from the paper. The repulpable silicone release liner permits recovery of paper fibers using conventional repulping and recycling processes.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of International Application No.PCT/US2017/051999, filed on Sep. 18, 2017, which claims priority under35 U.S.C. § 119(e) to U.S. Provisional Application No. 62/396,926, filedon Sep. 20, 2016, entitled “REPULPABLE SILICONE RELEASE LINER PAPER WITHDISSOLVABLE LAYER,” the entire disclosures of which are herebyincorporated herein by reference.

BACKGROUND OF THE INVENTION

Paper is a porous material composed of cellulose fibers that can bereadily penetrated by silicone coatings. For this reason, dense papers,such as glassine, parchment, and greaseproof papers, are used in orderto minimize the amount of silicone that must be applied to obtain therequired release characteristics of paper. Other low-porosity and smoothpapers used as the base stock for release papers are: coated publishingpapers, label face papers, non-coated supercalendered (SC) papers, andmachine glazed (MG) papers. Due to the high cost of silicone, papersrequiring minimal additions of silicone to their surface for release aretypically most desired. Porosity and smoothness may greatly impact theamount of silicone required.

Due to the large amount of paper that is used, a considerable amount ofwaste paper is collected and recycled. This waste paper containsreusable fiber if the fibers can be separated from their non-cellulosicimpurities during the recycling process. During known recyclingprocesses, collected wastepaper and water is added to a pulper, whichbreaks apart (disintegrates) the paper into individual fibers bymechanical action. This is followed by various stages of cleaning andscreening to obtain fibers that are as pure as possible to prevent orreduce problems that may otherwise occur during re-use in a papermakingprocess.

Release papers play a vital role in the fabrication ofpressure-sensitive adhesive (PSA) labels and tags. Release papers serveas a disposable base layer to carry the labels through conversion anddispensing. Release papers are also used in the manufacturing ofself-adhesive materials and components for tapes, industrial and graphicapplications. Methods for obtaining release papers for adhesives aretypically based on at least one of three known chemistries: silicone,chrome complexes, and extruded PE (polyethylene). However, backing paperthat has been made utilizing these known chemistries may createdifficulties during recycling of paper.

BRIEF SUMMARY OF THE INVENTION

One aspect of the present disclosure is a method making repulpablepaper. The method includes coating at least a portion of a sheet ofpaper with a water-soluble material. At least a portion of thewater-soluble material is coated with a silicone material to form asilicone release paper. The method may include providing label stockhaving adhesive on at least one side thereof, and releasably adheringthe label stock to the silicone release paper by bringing the adhesiveinto contact with the silicone material. The silicone material mayinitially comprise a silicone polymer, a crosslinker, and a catalyst.The silicone material may be selected from a group consisting ofsolvent-based silicone, water-based silicone, and solvent-less silicone.The water-soluble material may comprise sodium alginate and partiallyhydrolyzed polyvinyl alcohol. The water soluble material may comprise awater soluble polymer. The water soluble polymer may be selected fromthe group consisting of carboxymethylated cellulose, ethylated starch,and carboxylated soy protein. The water soluble material may form acoating having a coat weight of about 2.0-6.0 gsm. The silicone materialmay form a coating having a coat weight of about 2.0-100.0 gsm. Themethod may include repulping the paper after coating the paper withwater-soluble material and silicone material. Repulping the paper mayinclude bringing the water-soluble material into contact with water. Thepaper may be soaked in water for at least about 30 minutes or at leastabout 60 minutes. A hydrapulper machine may be utilized to repulp thepaper. The paper may be soaked in water prior to placing the paper intoa hydropulper machine. Repulping the paper may result in less than about15% rejects according to predefined criteria. The percent rejects maycomprise oven dried rejects divided by the product of the consistency ofthe paper and the total mass of the paper.

Another aspect of the present disclosure is a method of makingrepulpable paper. The method includes coating at least a portion of asheet of paper with a layer of dissolvable sacrificial material. Atleast a portion of the dissolvable sacrificial material is coated with arelease material having non-stick properties. The dissolvablesacrificial layer may be dissolved in water, and the release materialmay comprise silicone.

These and other features, advantages, and objects of the presentinvention will be further understood and appreciated by those skilled inthe art by reference to the following specification, claims, andappended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a schematic cross sectional view showing the layers of atypical silicone release paper;

FIG. 2 shows a condensation reaction between a polymer and a crosslinkerto form a cross linked silicone network;

FIG. 3 is a chart showing % rejects for various materials;

FIG. 4 is a chart showing tensile load for repulped samples utilizingdifferent materials;

FIG. 5 is a chart showing tensile extension for different materials; and

FIG. 6 is a graph showing Oxford Twin X XRF Silicone Content TestResults.

DETAILED DESCRIPTION

With reference to FIG. 1, a typical silicone release paper 1 includeslabel stock 2 that is adhered to a silicone layer 6 by adhesive 4.Silicone 6 is disposed on surface size 8 of a base sheet of paper 10.

With reference to FIG. 2, a typical silicone release coating includes asilicone polymer, a crosslinker, and a catalyst. A condensation reactionoccurs between the catalyst and silicone polymer liberating hydrogeninducing the formation of a cross-linked silicone network.

Silicone may be used to produce release papers due to its very lowsurface tension. The silicone molecule also has a low molecularpolarity, which facilitates binding or crosslinking of the silicone tothe cellulose fibers in paper. Three different types of silicones usedfor the manufacture of release papers are: solvent-based, water-based,and solvent-less. While the use of these materials provides performancefor a broad range of adhesives, release liners produced from thesematerials are not repulpable or recyclable. Chrome complexes represent apotential environmental hazard due to their chromium content and theresulting problems of heavy metals. PE extruded papers are also notrecyclable because the PE polymer cannot be disintegrated in a typicalrepulper.

Non-recyclable release liners result in significant cost becausenon-recyclable release liners must be sent to a landfill or incinerated.Silicone coatings are typically not recyclable because the siliconecannot be completely separated from the fibers. Even trace amounts ofsilicone in a recycle stream may cause spots on the resulting paperand/or build up on a paper machine, resulting in expensive down time forcleaning.

The present disclosure provides for the application of a water-solublesacrificial coating layer to a low porosity, smooth base paper, such asan MG paper, to provide repulpability of such papers. The water-solublecoating layer may be of sufficient thickness to completely cover thesurface of the paper to prevent penetration of the silicone into thenetwork of paper fibers. It will be understood that providing sufficientthickness does not necessarily imply a specific coverage. Completelycovering the surface of the fibers with a sacrificial coating layerpermits the removal of a silicone release coating applied over thesacrificial coating upon immersion in water. Specifically, theunderlying sacrificial layer is re-solubilized when the coated fibersare immersed in water, thereby permitting removal of the siliconerelease coating.

As discussed below, a sacrificial layer of sodium alginate and partiallyhydrolyzed polyvinyl alcohol may be utilized. However, other watersoluble polymers such as carboxymethylated cellulose, ethylated starch,and carboxylated soy protein could also be used. An advantage of themethod/technology of the present disclosure is the ability to producesilicone release papers that can be repulped using conventionalrepulping processing equipment without chemical additives, hence makingrecovery of the fibers economically feasible.

EXAMPLE

A commercial grade Duncote Ultra, Lightweight Coated, paper was used asthe base paper to fabricate test samples. For the experiment, first andsecond samples of the paper were coated with first and second differentwater-soluble polymers, respectively, using different Myer rods toobtain approximately the same coat weight (3.0-4.0 gsm) on the first andsecond test samples. The first and second water soluble polymers usedwere sodium alginate, S-15-C (“alginate”) (available from SNP Inc.) andPVA, Selvol 2035 (available from Sekisui America Corporation, Secaucus,N.J.), respectively. The alginate was formulated to an 8.0% solution (byweight) by slowly adding the alginate to water and allowing it to mixfor about 30 minutes. The PVA was formulated to a 28% solution (byweight) by adding the dry powder into room temperature water under amixer. The contents were then heated on a steam table under agitationuntil the PVA was completely dissolved. Use of #30 and #6 rod for thealginate and PVA, respectively, enabled the desired target coat weightto be obtained for the first and second test samples. The actual coatweights applied were 3.84 (+0.19) gsm for the alginate and 4.05 (+0.14)gsm for the PVA. Coat weights were measured gravimetrically using aLabwave CEM solids analyzer.

The first and second test samples were then coated with two differentcommercially available silicones using a 1.5 mil Byrd applicator. Thefirst test sample was coated with Sylgard®-184 (Dow Corning) siliconemixed in the recommended 10:1 ratio of silicone to cross-linker, and thesecond test sample was coated with Wacker 944 Dehesive® 944 siliconerelease coating (available from Wacker Chemie AG) using a low SiHformulation consisting of 60.35% Dehesive® 944, 37.59% toluene, 0.3%Crosslinker V90, and 1.76% Catalyst C 05. To cure the silicone coatings,the first and second test samples were dried in a thermal oven at 257°F. for 20 minutes. After drying, a silicone cure test was performed bymeasuring the tape adhesion properties using Scotch® brand cellophanetape 610 to ensure a complete cure had been achieved. All test samplesshowed no degree of detackification. After being dried, the first andsecond test samples were conditioned in a TAPPI standard control room(50% RH and 85° F.) for 24 hours and the coat weights of the appliedsilicone was measured. The high coat weight of the Sylgard® incomparison to the Wacker product is likely due to the extremely highviscosity of the Sylgard®.

The first and second test samples were then separately repulped using aMaelstrom laboratory hyrapulper (available from Maelstrom AdvancedProcess Technologies LTD, Derbyshire, UK). As a control sample, acommercial silicone release paper (label stock removed) was alsorepulped. The repulping was performed at a 6% consistency by tearing aknown weight of each test sample into an appropriate amount of watercontained within the Maelstrom hydrapulper. During addition of the testsamples, the conditions of the hydrapulper were initially maintained at300 rpm and 140° F. For each test sample, the mixing speed was increasedto 400 rpm, approximately half way through the addition of samples(i.e., after about 15 minutes) to the hydrapulper. Upon completion ofeach test sample addition, the speed of the hydrapulper was increased to700 rpm and held at this speed for 20 minutes. After 20 minutes thehydrapulper was discharged and the consistency of the repulped materialwas measured.

A portion of each test sample was then taken and weighed for screening.Screening was performed using a Johnson® 6-cut screen (available fromJohnson Screens®) to separate and collect the acceptable andnon-acceptable waste materials (streams) for weighing. The first andsecond test samples were allowed to run in the 6-cut screen until noapparent fibers were present in the accepts stream. To determine theweight of rejects, the collected reject test samples were poured into aBuchner funnel containing a pre-weighed oven dried filter Whatman filterpaper. The reject test sample and filter paper were then dried on a hotplate until bone dry, at which time the combined weight of the rejectsample and filter was measured. The % rejects was then calculated bydividing the weight of the oven dried rejects (ODR) of each test sampleby the consistency of the stock multiplied times the total mass of thetest stock fed into the 6-cut. It will be understood that the % rejectsis found using an equation that divides the weight of over dried fiberin reject stream by the total mass of oven dried fiber in the inputstream. The consistency is the weight of oven dried fiber/oven driedfiber+water. All terms are defined by industry standards and the 15% isset by the RPTA. Next, the consistencies of the accepts were measured toenable the mass required to produce a 1.2 g oven dried TAPPI standardhand sheet to be determined.

TAPPI standard hand sheets were then made for each test sample setfollowing TAPPI standard method T-205 procedures. After allowing thehand sheets to condition for a minimum of 24 hours the weight of eachtest sample was measured using a Mettler (AE 260) 3-point scale toenable the calculation of tensile index values. Tensile tests wereperformed according to TAPPI standard T-494 using an Instron TensileTester. Peak load and extension were tested. Ten tests were performedfor each sample. The water absorptivity of each sample was also measuredusing a FTA dynamic contact angle apparatus. Both sides of the sampleswere tested utilizing an Oxford Instruments Lab-X 3000 EDRF (X-Ray) unitcalibrated for silicone coat weights.

For this testing, each set of test samples was corrected using theDuncote base paper as the control to determine baseline of Si present,from clay, prior to silicone coating. The roughness and permeability ofthe samples prior to application of the silicone were measured using aParker Print-Surf, Model No. ME-90 instrument. For roughness readings, asoft-backing plate was used at both 500 and 1000 kPa.

Test Results and Discussion

Contact angle measurements of the hand sheets made from the acceptsstream failed due to water penetrating the substrate too quickly toenable image capture, and hence, analysis of contact angle changes fromsample to sample. Results of the base sheet properties are shown inTable 1.

TABLE 2 Paper Properties (Base Sheet Duncote Paper) Surface EnergyRoughness Caliper Permeability (dyne/cm) (μm) (μm) (mL/min) 49.3 ± 0.50.83 ± 0.03 74.0 ± 1.5 2.11 ± 0.08

The results of the reject percentages collected from each repulpingstudy are shown in FIG. 3.

The results show a decrease in the rejects for the Wacker siliconecoated papers with sacrificial layer present. The % rejects was lowerfor the PVOH pre-coated papers than the alginate pre-coated papers. Thiscould be due to the higher roughness of the alginate samples caused bygreater shrinkage during drying do to its lower solids. The results forthe Sylgard were completely different. As shown, the % rejects increasedfor the samples pre-coated with alginate and PVOH. Upon furtherinvestigation of these samples, it was observed that the majority of therejects was silicone. The high coat weight of Sylgard in comparison tothat of the Wacker may be the cause of the differences obtained.

To better understand these findings, an additional repulping study wasperformed. In this study a soaking step (1 hour) was added prior toadding the material into the hydrapulper. After soaking, any materialsfloating on the surface were skimmed off, subsequently, the same pulpingprocedures were followed as previously performed. The significantreduction in rejects for the Sylgard pre-coated samples confirm that themajority of the rejects were from the higher amount of Sylgard additionlevel, in comparison to the Wacker. By pre-soaking the samples, thesacrificial layer performed as desired, enabling the release of thesilicone. The lower level of rejects obtained brought the percentrejects to a level to meet the Recycled Paperboard Technical Association(RPTA) requirements (less than 15%) to be considered a repulpableproduct.

The results of the tensile tests are shown in FIGS. 4 and 5, which showthe tensile load and extension results, respectively. The results show adecrease in load strength of the samples relative to the commercialsilicone release paper. However, only the Sylgard sample shows asignificant difference prior to the introduction of the sacrificiallayer. With the introduction of the sacrificial layer all samples, withthe exception of the alginate Sylgard sample, showed a significantdecrease in load strength versus that of the commercial paper. The loadstrength of the pre-soaked and Duncote control samples shows that theaddition of the pre-soaking step was effective in removing a majority ofthe silicone. The lowest load strength was found for the Wacker coatedsamples. These low strength values are attributed to the low coat weightand insolubility of the Wacker silicone, enabling the largest amount ofsilicone to make it through the accept stream and into the handsheets.

The tensile extension results (FIG. 5), show similar trends as thetensile load results. Here, however, all extensional values are lower incomparison to the commercial silicone paper. These results show theSylgard coated samples having larger extensional values as compared tothat of the Wacker samples, and the alginate/Sylgard samples having thehighest average values of all the coated samples.

From FIG. 6 it may be seen that the Commercial silicone release paperhas the highest silicone content present. The handsheets having thelowest silicone content are those of the Duncote/Wacker andDuncote/Alginate/Sylgard. A decrease in silicone content may be seenbetween the Duncote/Sylgard and Duncote/Alginate/Sylgard samples,showing the alginate sacrificial layer decreased the amount of siliconecarried over into the accepts stream.

The large standard deviation seen with the Duncote/Polyvinyl Alcohol/Wacker sample could be due to sampling error. It is believed thatwithout this error the average value for this sample would align withthat of the Duncote/Polyvinly Alcohol/Sylgard test sample. The twolowest test sample averages are that of the Duncote/Wacker, andDuncote/Alginate/Wacker. These test samples may have the lowest averagesdue to the lower coat weight and ability to flow through the wastestream presented from the Wacker coating. With theDuncote/Alginate/Sylgard sample the sacrificial coating may becompleting separation of the PDMS from the paper fibers, therebyallowing the (PDMS) film to be removed during the recycling process.

In the example discussed above, the application of a water-solublesacrificial pre-coat prior to the application of a silicone releasecoating resulted in the majority of the silicone being separated andremoved during the repulping process. The lower level of rejectsobtained by first applying the sacrificial layer decreased the percentrejects to a level that meets the Recycled Paperboard TechnicalAssociation (RPTA) requirements (less than 15%) to be achieved. Thus,the application of a water soluble layer successfully enabled a siliconerelease paper product to be repulped by use of conventional repulpingand recycling processing methods.

APPENDIX

Tensile Index Data Load/Grammage Commercial Silicone Release 0.05Duncote 0.04 Duncote_Sylgard 0.04 Duncote_Wacker 0.03Duncote_Alginate_Sylgard 0.04 Duncote_PVOH_Sylgard 0.03Duncote_Alginate_Wacker 0.03 Duncote_PVOH_Wacker 0.02Duncote_Alginate_Sylgard (Pre Soaked) 0.03

The invention claimed is:
 1. A method of making repulpable paper, themethod comprising: coating at least a portion of a sheet of paper with awater-soluble material; coating at least a portion of the water-solublematerial with a silicone material to form a silicone release paper. 2.The method of claim 1, including: providing label stock having adhesiveon at least one side thereof; and releasably adhering the label stock tothe silicone release paper by bringing the adhesive into contact withthe silicone material.
 3. The method of claim 1, wherein: the siliconematerial initially comprises a silicone polymer, a crosslinker, and acatalyst.
 4. The method of claim 1, wherein: the silicone material isselected from the group consisting of solvent-based silicone,water-based silicone, and solvent-less silicone.
 5. The method of claim1, wherein: the water-soluble material comprises at least one of sodiumalginate and partially hydrolyzed polyvinyl alcohol.
 6. The method ofclaim 1, wherein: the water-soluble material comprises a water solublepolymer.
 7. The method of claim 6, wherein: the water soluble polymer isselected from the group consisting of carboxymethylated cellulose,ethylated starch, and carboxylated soy protein.
 8. The method of claim1, wherein: the water soluble material forms a coating having a coatweight of about 2.0-6.0 gsm.
 9. The method of claim 1, wherein: thesilicone material forms a coating having a coat weight of about2.0-100.0 gsm.
 10. The method of claim 1, including: repulping the paperafter coating the paper with water-soluble material and siliconematerial.
 11. The method of claim 10, wherein: repulping the paperincludes bringing the water-soluble material into contact with water.12. The method of claim 11, wherein: the paper is soaked in water for atleast about 30 minutes.
 13. The method of claim 12, wherein: the paperis soaked in water for at least about 60 minutes.
 14. The method ofclaim 10, wherein: a hydrapulper machine is utilized to repulp thepaper.
 15. The method of claim 14, including: soaking the paper in waterprior to placing the paper into a hydrapulper machine.
 16. The method ofclaim 10, wherein: repulping the paper results in less than 15 percentrejects according to predefined criteria.
 17. The method of claim 15,including: skimming off material floating on the surface after soakingthe paper.
 18. A method of making repulpable paper, the methodcomprising: coating at least a portion of a sheet of paper with a layerof dissolvable sacrificial material; and coating at least a portion ofthe dissolvable sacrificial material with a release material havingnon-stick properties.
 19. The method of claim 18, wherein: thedissolvable sacrificial layer dissolves in water.
 20. The method ofclaim 18, wherein: the release material comprises silicone.